This invention relates to power converters which transform electric energy from a polyphase a.c. source to an a.c. of other voltage, frequency, or waveform or to d.c. Or in reverse, it can receive electric energy from an a.c. or d.c. source and transform it to polyphase a.c. power of other voltage, frequency or waveform.
Static power converters, as just described, are well known in the art. One representative group of this kind is the antiparallel, full wave, three phase thyristor bridge. This bridge is, primarily, used for controlled a.c. to d.c. conversion; it has found wide application for that purpose and is presently one of the important implements of electric power technology. One area of application is the controlled drive of d.c. machines used in elevators, rolling mills and other cases which require motor speed control and reversal of direction of rotation. The above named thyristor bridge is also used as one functional element in tandem with a four quadrant inverter to form one four quadrant converter. This converter transforms polyphase a.c. power with one voltage and frequency to a.c. power with another voltage and frequency. The above named thyristor bridge performs, therefore, a valuable and needed function in present day technology.
The functional principles of the thyristor bridge impose certain constraints on its use. A brief summary of some of these limiations follows. The bridge (1) requires protection against the occurrence of excessive currents in the conducting thyristors which can be caused by even temporary over loads associated with dynamic load conditions. The therewith associated protection requires (2) subordination of load performance to the limitations of the bridge and (3) utilization of individual thyristor turn-off circuits which in turn require doubling of the number of 12 thyristors for a three phase a.c. supply line to 24 thyristors with comparable v-i ratings. The bridge requires, furthermore, (4) fuse protection in case the load current should rise faster than compatible with the above referred to thyristor turn-off mechanisms. Reaction speed of the named thyristor bridge is (5) limited to approximately 2.8 milliseconds, being one-sixth of the 16.66 ms. period of a 60 Hz system and is actually slower because of the facts cited under (1) above. Use of thyristor bridges of the named kind requires in many cases utilization of low pass filters in the input or in the output circuit of the bridge, or both. It is the purpose of the ( 6) input filter to prevent pollution of the a.c. supply network caused by application of the well-known principle of phase angle control which causes distortion in the supply voltage waves. This distortion can interfere with the utilization of the same supply line by other users which may require limitation of harmonic distortions in that line to prescribed tolerances. Low pass filters with a cut-off frequency above, but near 60 Hz are required for that purpose in the individual lines of the polyphase network. The purpose of the (7) output filter is to smooth the thyristor switched load current to prevent unnecessary ohmic losses in the load which would be caused by the presence of significant higher harmonic components in this current. Another problem is associated with the (8) power factor, as viewed from the supply line. This power factor depends on the characteristics of input and output filters as cited under limitations (6) and (7) above, respectively; it also depends on the conditions of loading, which, in turn, determines the cut in phase angle of the thyristors of the bridge. This power factor is, in many cases, unfavorable in the most important "active region" of control of the control phase angle .alpha..sub.min &lt; .alpha. &lt;.alpha..sub.max for given conditions. A low power factor in the a.c. supply line results in a poor utilization of the feeding generator and of the therewith associated means of power transmission, due to additional ohmic losses in these power system components. The same low power factor causes also added losses in the bridge, the therewith associated filters, and in the load if the filters are inadequate, meaning too light.
Virtually, all present day polyphase a.c. powered four quadrant converters apply the principle of phase angle control, described above, with reference to the thyristor bridge. They suffer, therefore, from the constraints and disadvantages as described above.